HT68F13 datasheet(17/161 Pages) HOLTEK

17 / 161 page

For instructions involving branches, such as jump or call instructions, two machine cycles are required

to complete instruction execution. An extra cycle is required as the program takes one cycle to first

obtain the actual jump or call address and then another cycle to actually execute the branch. The

requirement for this extra cycle should be taken into account by programmers in timing sensitive

applications.

ProgramCounter

During program execution, the Program Counter is used to keep track of the address of the next

instruction to be executed. It is automatically incremented by one each time an instruction is executed

except for instructions, such as

²JMP² or ²CALL² that demand a jump to a non-consecutive Program

Memory address. Only the lower 8 bits, known as the Program Counter Low Register, are directly

addressable by the application program.

When executing instructions requiring jumps to non-consecutive addresses such as a jump instruction,

a subroutine call, interrupt or reset, etc., the microcontroller manages program control by loading the

required address into the Program Counter. For conditional skip instructions, once the condition has

been met, the next instruction, which has already been fetched during the present instruction

execution, is discarded and a dummy cycle takes its place while the correct instruction is obtained.

Device

ProgramCounter

HighByte

LowByte

(PCL Register)

HT68F13

PC9,PC8

PCL7~PCL0

HT68F14

PC10~PC8

HT68F15

PC11~PC8

ProgramCounter

The lower byte of the Program Counter, known as the Program Counter Low register or PCL, is

available for program control and is a readable and writeable register. By transferring data directly into

this register, a short program jump can be executed directly, however, as only this low byte is available

for manipulation, the jumps are limited to the present page of memory, that is 256 locations. When such

program jumps are executed it should also be noted that a dummy cycle will be inserted. Manipulating

the PCL register may cause program branching, so an extra cycle is needed to pre-fetch.

Stack

This is a special part of the memory which is used to save the contents of the Program Counter only.

The stack has multiple levels depending upon the device and is neither part of the data nor part of the

program space, and is neither readable nor writeable. The activated level is indexed by the Stack

Pointer, and is neither readable nor writeable. At a subroutine call or interrupt acknowledge signal, the

contents of the Program Counter are pushed onto the stack. At the end of a subroutine or an interrupt

routine, signaled by a return instruction, RET or RETI, the Program Counter is restored to its previous

value from the stack. After a device reset, the Stack Pointer will point to the top of the stack.

If the stack is full and an enabled interrupt takes place, the interrupt request flag will be recorded but

the acknowledge signal will be inhibited. When the Stack Pointer is decremented, by RET or RETI, the

interrupt will be serviced. This feature prevents stack overflow allowing the programmer to use the

structure more easily. However, when the stack is full, a CALL subroutine instruction can still be

executed which will result in a stack overflow. Precautions should be taken to avoid such cases which

might cause unpredictable program branching.

HT68F13/HT68F14/HT68F15

EnhancedI/OFlashType MCU

Rev.1.10

February9,2011

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